Technology update

Sep 4, 2008

Tiny probe looks good on camera

Evanescent electromagnetic field illumination is paving the way for high-resolution imaging of soft-samples at fast frame rates in liquid. Researchers from the University of Bristol, UK, in collaboration with the Hanover Medical School, Germany, are using the system to monitor the displacement of vertically mounted SPM probes that have been tailor-made for non-contact imaging of biomolecules and membranes.

"We hope to gain new insights into the dynamic behaviour of fundamental biological processes such as protein translocation," Loren Picco of Bristol University's nanophysics and soft matter group told nanotechweb.org. "The results could be important in the development of antibiotics."

The instrument's custom-milled probes have a very high resonance frequency to stiffness ratio and measure just 200 nm in width. In fact, the cantilevers are so narrow that conventional detection schemes based on the so-called optical lever method cannot be applied. To get round the problem, the group relies instead on the scattering of an evanescent wave to measure the displacement of its probe.

The evanescent wave is located just above the instrument's transparent sample surface and is produced using an internally reflected laser beam (see diagram). When the probe tip enters the evanescent field, it scatters the light and the image of the tip is projected on to a four-sector photodiode.

"The next steps are to increase the resonant frequencies of the mechanical components and to improve the bandwidth and response of the electronics," said Picco. "The combined effect will be to increase the rate at which we can safely and stably scan the sample surface, because the improvements will allow the instrument to respond more quickly to the variations in topography."

The researchers presented their work in a special issue of Nanotechnology.